The invention pertains to a doping mixture for coating of semiconductor substrates that are then fed to high temperature treatment to form a doped layer. The invention also concerns a method for production of such a doping mixture, as well as its use.
The conductivity of semiconductors is generally increased by the fact that foreign atoms with a slightly higher or lower valence than that of the semiconductor material are incorporated in its crystal structure. In principle, this method can be applied to all types of semiconductors, but it has gained particular significance in the field of doping of silicon-based semiconductors. If, for example, pentavalent phosphorus is introduced to a crystal lattice of silicon atoms, the phosphorus atom occupies a location in the crystal lattice in place of a silicon atom. Since phosphorus has five valence electrons and only four are required for bonding with its four neighboring silicon atoms, one electron is left without a fixed bond. This electron is available for current conduction without expenditure of energy. A material doped in this way is called n-doped and is n-conducting. The introduction of an atom with lower valence than that of the semiconductor leads accordingly to an electron deficit, which also leads to an increase in conductivity. Such materials are called p-doped and are p-conducting.
Diffusion of phosphorus in the silicon wafer during formation of emitters for solar cells frequently occurs in a furnace with a so-called inline transport system. The wafers are initially coated with a phosphorus-containing layer and then treated at temperatures in a range from about 800° C. to 1000° C. The phosphorus atoms diffuse at these high temperatures from the coating into the wafer and thus form the emitter structures. Although this so-called inline diffusion is widespread, there is still no predominant technique for depositing the phosphorus-containing layer on the wafer before the diffusion step.
Inline diffusion has many advantages over the still widespread and commonly used traditional POCl3 process in the prior art. In the context of such a process, POCl3 is ordinarily applied in a preparation appropriate for this as a layer to the wafer and then heat-treated in a quartz tubular furnace. The inline method has a large number of advantages relative to this conventional method. The wafer is moved less frequently, from which a reduced hazard of wafer breakage results. However, different shortcomings have thus far been attached to the inline method that hampered utilization of the efficiency that this method can offer.
An important step for efficiency of a doping method exists in applying the doping source to the semiconductor being doped. Some methods for exposing the semiconductor to a corresponding dopant are already used, for example, in the microelectronics industry. Examples of such methods are chemical vapor deposition (CVD), in which, for example, gaseous phosphorus (for example, phosphane or phosphine) is evaporated on a surface being doped. Spin-coating, in which a sol-gel (a solution that contains the phosphorus silicate polymer molecules dissolved or suspended in an organic solvent) mixed with a phosphorus compound is often applied to the surface being doped and this placed in rotation, so that the solution or suspension is uniformly distributed on the surface, is also used as a method for applying a dopant to a semiconductor surface.
Excellently uniform and pure phosphorus silicate glasses can be obtained with the CVD method, but performance of the method requires complex equipment and precise process control. Moreover, hazardous materials are often used. The described spin-coating method does represent a much simpler alternative, but the drawbacks of the method are far more serious than the simple execution. Thus, in the spin-coating method, only about 2 to 5% of the employed material is actually used on the semiconductor surface for doping. This drastically increases the cost of the process overall. There is also the risk that during spin-coating, small regions uncoated by dopant will remain, and this is a particular hazard in textured semiconductor surfaces.
As another possibility for application of a dopant to a semiconductor surface, the screen-printing method was already used more than 20 years ago. This method has the advantage that formation of selected emitters is possible, in particular, based on the high viscosity of the applied material. However, in this method, pressure is exerted on the wafer, so that the risk of wafer breakage is significantly increased. Moreover, the employed materials contain a large amount of organic ingredients that burn during drying and diffusion of the dopant. This produces the potential problem that the organic residues will adhere strongly to the semiconductor surface, so that they remain permanently on the surface. Dipping the wafers in a dopant solution, in which the wafers are positioned on a corresponding conveyor belt that passes through a corresponding dopant solution, are among additional possible methods for application of a dopant to semiconductor surfaces. Rolling of a paste or liquid containing the corresponding dopant with a roller, which enters into contact with the wafer with slight pressure, is also possible. A common feature of both methods is that the formation of a reproducible thin and uniform layer of dopant on the surface is connected with extraordinarily serious difficulties.
Various attempts have been made to apply a dopant by spray application to the semiconductor surface. However, this turned out to be extraordinarily difficult, because of the material properties of ordinary dopant mixtures, since generally a sufficiently homogeneous distribution of the dopant mixture on the semiconductor surface could not be achieved. Experiments with phosphoric acid as dopant involved, in particular, the use of phosphoric acid vapor on wafer surfaces that were held at room temperature. The use of hot phosphoric acid and phosphoric acid vapor, however, is extremely problematical, because of the strong corrosion caused by hot phosphoric acid. Deposition of phosphoric acid from aqueous solution, however, was only insufficient, because of the surface condition of the semiconductor surface. Application of aqueous solutions generally led to the formation of isolated drops on the surface, which did not permit uniform doping.
EP 1 414 082 A2 discloses the use of aqueous solutions with doping surfactants for doping of organic and inorganic semiconductors.
Consequently, there was a demand for a method, with which a dopant can be simply applied quickly and in automated fashion to semiconductor surfaces, especially the surface of wafers. In particular, there was a demand for a method that permits such application of a dopant with the least possible material loss and the most reproducible and uniform possible result.